Re: [PATCH net] net: Remove redundant calls of sk_tx_queue_clear().

[Kuniyuki Iwashima <kuniyu@amazon.co.jp>]

From:   Eric Dumazet <edumazet@google.com>
Date:   Wed, 27 Jan 2021 15:54:32 +0100
> On Wed, Jan 27, 2021 at 1:50 PM Kuniyuki Iwashima <kuniyu@amazon.co.jp> wrote:
> >
> > The commit 41b14fb8724d ("net: Do not clear the sock TX queue in
> > sk_set_socket()") removes sk_tx_queue_clear() from sk_set_socket() and adds
> > it instead in sk_alloc() and sk_clone_lock() to fix an issue introduced in
> > the commit e022f0b4a03f ("net: Introduce sk_tx_queue_mapping"). However,
> > the original commit had already put sk_tx_queue_clear() in sk_prot_alloc():
> > the callee of sk_alloc() and sk_clone_lock(). Thus sk_tx_queue_clear() is
> > called twice in each path currently.
> 
> Are you sure ?
> 
> I do not clearly see the sk_tx_queue_clear() call from the cloning part.
> 
> Please elaborate.

If sk is not NULL in sk_prot_alloc(), sk_tx_queue_clear() is called [1].
Also the callers of sk_prot_alloc() are only sk_alloc() and sk_clone_lock().
If they finally return not NULL pointer, sk_tx_queue_clear() is called in
each function [2][3].

In the cloning part, sock_copy() is called after sk_prot_alloc(), but
skc_tx_queue_mapping is defined between skc_dontcopy_begin and
skc_dontcopy_end in struct sock_common [4]. So, sock_copy() does not
overwrite skc_tx_queue_mapping, and thus we can initialize it in
sk_prot_alloc().

[1] sk_prot_alloc
https://github.com/torvalds/linux/blob/master/net/core/sock.c#L1693

[2] sk_alloc
https://github.com/torvalds/linux/blob/master/net/core/sock.c#L1762

[3] sk_clone_lock
https://github.com/torvalds/linux/blob/master/net/core/sock.c#L1986

[4] struct sock_common
https://github.com/torvalds/linux/blob/master/include/net/sock.h#L218-L240


> In any case, this seems to be a candidate for net-next, this is not
> fixing a bug,
> this would be an optimization at most, and potentially adding a bug.
> 
> So if you resend this patch, you can mention the old commit in the changelog,
> but do not add a dubious Fixes: tag

I see.

I will remove the tag and resend this as a net-next candidate.

Thank you,
Kuniyuki


> >
> > This patch removes the redundant calls of sk_tx_queue_clear() in sk_alloc()
> > and sk_clone_lock().
> >
> > Fixes: 41b14fb8724d ("net: Do not clear the sock TX queue in sk_set_socket()")
> > CC: Tariq Toukan <tariqt@mellanox.com>
> > CC: Boris Pismenny <borisp@mellanox.com>
> > Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
> > Reviewed-by: Amit Shah <aams@amazon.de>
> > ---
> >  net/core/sock.c | 2 --
> >  1 file changed, 2 deletions(-)
> >
> > diff --git a/net/core/sock.c b/net/core/sock.c
> > index bbcd4b97eddd..5c665ee14159 100644
> > --- a/net/core/sock.c
> > +++ b/net/core/sock.c
> > @@ -1759,7 +1759,6 @@ struct sock *sk_alloc(struct net *net, int family, gfp_t priority,
> >                 cgroup_sk_alloc(&sk->sk_cgrp_data);
> >                 sock_update_classid(&sk->sk_cgrp_data);
> >                 sock_update_netprioidx(&sk->sk_cgrp_data);
> > -               sk_tx_queue_clear(sk);
> >         }
> >
> >         return sk;
> > @@ -1983,7 +1982,6 @@ struct sock *sk_clone_lock(const struct sock *sk, const gfp_t priority)
> >                  */
> >                 sk_refcnt_debug_inc(newsk);
> >                 sk_set_socket(newsk, NULL);
> > -               sk_tx_queue_clear(newsk);
> >                 RCU_INIT_POINTER(newsk->sk_wq, NULL);
> >
> >                 if (newsk->sk_prot->sockets_allocated)
> > --
> > 2.17.2 (Apple Git-113)
> >